The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by an optical gain medium in the laser cavity and a method for providing optical feedback. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
In semiconductor lasers, a semiconductor active region serves as the gain medium. Semiconductor lasers may be diode (bipolar) lasers or non-diode, unipolar lasers such as quantum cascade (QC) lasers. Semiconductor lasers are used for a variety of industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam. For monolithic designs, the optical feedback is typically provided by a reflector or reflectors external and/or adjacent to the optical gain medium or some combination of feedback mechanisms. For example, in Fabry-Perot or Vertical-Cavity Surface-Emitting Laser (VCSEL) lasers a set of mirrors or cleaved facets that bound the optical gain medium may provide the optical feedback. In distributed feedback (DFB) lasers, a distributed reflector along the gain medium may provide the feedback. The distributed reflector may be a Bragg reflector (i.e., Bragg grating). A distributed Bragg reflector (grating) may also be used as an external reflector. In this case, a Bragg grating or gratings may be at or near the ends of the gain medium. A laser with a distributed Bragg reflector as an external reflector is known as a Distributed Bragg Reflector (DBR) laser. In some embodiments, a DFB laser may have external reflectors in addition to a distributed reflector.
A semiconductor laser with simple broad wavelength external reflectors will generally operate at several wavelengths, or optical modes. For some applications such as optical fiber communication or chemical sensing, operation at a single optical mode is strongly preferred. In order to achieve single mode operation with external reflectors, those external reflectors must provide very wavelength-selective feedback, which adds significantly to the complexity and cost.
A distributed reflector is often used as a convenient and relatively inexpensive method for making a laser operate with a single optical mode. In its simplest configuration, with a uniform distributed reflector and no external reflectors, two laser modes are supported. There are two common approaches for achieving single mode operation. One approach is to use one or more external reflectors in addition to the distributed reflector and the other is to put one or more phase shifts within the distributed reflector. The use of external reflectors can allow much more light to be emitted from one of the two ends (facets) of the laser, which can be beneficial for example in coupling light into an optical fiber with greater efficiency. An external reflector is inherently formed at the interface between the semiconductor and air. Increasing or decreasing the reflectance is typically achieved by applying optical coatings on the laser ends.
One disadvantage of using external reflectors in conjunction with a distributed reflector is that the phase shift (i.e., the distance) between the external reflector and the distributed reflector affects the operating characteristics of the laser. Some of the changes in the laser characteristics are undesirable (e.g., insufficient suppression of the second laser mode). For a typical semiconductor DFB laser, a distance on the order of nanometers can result in a significant change in the phase. Generally, this distance cannot be completely controlled. Compensating for this phase can be achieved with a tuning mechanism such as multiple-section current injection which adds significantly to the cost and the complexity and the device. Another common approach is to discard the lasers with the undesirable operating characteristics. This approach will add to the cost depending on the range of acceptable operating characteristics for the intended application.
The use of phase shifts in the distributed reflector can allow for single mode operation with less variability in the laser operating characteristics. Because this approach is used with antireflection coatings, the power emits more equally from both laser facets, leading to a lower usable power and efficiency. In addition the optical intensity within the laser cavity can vary strongly. This non-uniformity in the optical intensity can result in mechanisms such as localized gain saturation, spatial hole burning, and localized heating which degrade the laser performance.
Several approaches have been proposed to improve the optical non-uniformity. Multiple phase shifts and distributed phase shifts improve the uniformity of the optical intensity. One example of a single mode DFB laser having an improved phase-shift section is disclosed in U.S. Pat. No. 6,608,855, which is herein incorporated by reference. Varying the pitch or the coupling strength of the grating or using multi-section current injection have been proposed as ways to improve phase-shifted DFB lasers, but are more complex if not impractical to implement. In addition, gratings with modulated coupling strength have a degraded side mode suppression ratio (SMSR), which is a measure of the suppression of the second strongest lasing mode. All of these approaches use no external reflector and still suffer from reduced efficiency due to backside losses. Another approach which has been proposed for improving the uniformity of DFB lasers with external reflectors is a sampled grating. While this approach should be simple to fabricate, the improvement in the optical field uniformity is insufficient.
DFB lasers may include both continuous wave (CW) and directly modulated (DM) semiconductor lasers. A CW DFB laser may include a light generating portion (e.g., cavity) and a modulation portion. A continuous, approximately constant, input current, at or near lasing threshold, may be supplied to the light generating portion. A modulation current may be supplied to the modulation portion. Accordingly, a CW DFB laser may generate a continuous light output in the light generating portion, which is then modulated by the modulation current in the modulation portion.
A DM DFB laser may generate and modulate light in the same portion (e.g., cavity). An input current may be supplied that has two components. The first component may be a threshold current sufficient for lasing. The second component may include a bias current and a modulation current. Rather than a continuous light output that is then modulated, the input current that drives the laser is itself modulated. For example, laser drive circuitry may include one or more current sources that are configured to provide threshold current sufficient for lasing, bias current for establishing an operating point above the threshold and a modulation current. Accordingly, the input current to the laser is modulated resulting in modulated light output.
A DM DFB laser may be more susceptible than a CW DFB to nonlinearities (e.g., optical non-uniformities) that may affect the light output power versus input current relationship of the laser cavity (e.g., slope efficiency). In the case of the CW DFB laser, the input current to the laser cavity may be approximately constant (i.e., the laser light output is modulated external to the laser cavity). Accordingly, the cavity's nonlinearities may not significantly affect the light output power. The DM DFB laser is sensitive to these nonlinearities because the input current includes the modulation current. It may therefore be desirable to reduce such nonlinearities for DM DFB lasers.